Conduction system for thin film and hybrid integrated circuits

ABSTRACT

A metallization scheme for interconnection of elements in thin film and hybrid circuits is described. A thin layer of titanium is first formed, preferably by evaporation or sputtering, on the surface of the insulating substrate. A thin layer of copper is then formed in the same manner over the titanium layer. This is followed by electroplating of copper to a desired thickness onto selected portions of the Ti-Cu multilayer. Successive layers of nickel and gold are then selectively electroplated onto the plated copper regions. An additional layer of palladium may also be included between the titanium and copper layers for improved adhesion. The Ti-Cu-Ni-Au metallization system has been found unusually compatible with the major processing requirements of thin film circuits, for example, thermocompression bonding, soldering, via-hole coverage and resistor stabilization.

BACKGROUND OF THE INVENTION

This invention relates to thin film and hybrid integrated circuits, andin particular to a conduction system for interconnecting elements insaid circuits.

Thin film and hybrid integrated circuits presently enjoy extensive usein a wide variety of applications, for example, filter circuits andmemories for switching and transmission systems. The most widely usedmetal interconnection system used for such applications is atitanium-palladium-gold multilayer structure. The gold layer providesthe major current-carrying load while also serving as a good bondinglayer. While such a system has performed satisfactorily, greatquantitites of gold are required. Typically, the gold layer isapproximately 50,000 A thick and must be formed over a substantial areaof the interconnect pattern. With the rising cost of gold, the cost ofproducing such circuits can become excessive.

Finding a replacement for such a system is an exceedingly difficulttask. The system must not only provide good conduction and adherence tothe substrate, but must also be compatible with the various processingsequences necessary for circuit fabrication, such as thermo-compressionbonding, soldering, resistor and capacitor fabrication andstabilization, annealing, etc. The various components of the system mustalso be compatible, i.e., nonreactive, with each other.

It is therefore a primary object of the invention to provide a metalinterconnection system which is less expensive than the presently usedsystem and at the same time is compatible with the myriad processingrequirements of thin film and hybrid circuits.

SUMMARY OF THE INVENTION

This and other objects are achieved in accordance with the inventionwhich provides a metal combination comprising layers oftitanium-copper-nickel-gold for interconnection. Copper serves as theprimary conductor and gold as a bonding layer, thus significantlyreducing the thickness of gold required. Titanium provides good adhesionto the insulating substrate and nickel serves as a diffusion barrierbetween the copper and gold layers. In addition, a layer of palladiummay be provided for greater adhesion between the titanium and copperlayers. In accordance with one embodiment of the method of theinvention, thin layers of titanium and copper are successively formedover substantially the entire surface of the substrate by evaporation orsputtering. Additional copper is then electro-plated onto the evaporatedor sputtered copper in the areas which will comprise the interconnectpattern. A layer of nickel is then electroplated onto the plated coppersurface, followed by electroplating gold on the entire nickel surfaceor, alternatively, only on the areas which will comprise the bondingpads. The areas of titanium and copper which are not covered by theplated metals can then be etched to define the interconnect pattern.

BRIEF DESCRIPTION OF THE DRAWING

These and other features of the invention will be delineated in detailin the description to follow. In the drawing:

FIG. 1 is a flow diagram illustrating the fabrication steps inaccordance with two alternative embodiments of the invention;

FIG. 2 is a perspective view of a thin film circuit which has beenfabricated in accordance with one embodiment of the invention; and

FIG. 3 is an enlarged cross-sectional view taken along line 3--3 of FIG.2.

DETAILED DESCRIPTION OF THE INVENTION

The method in accordance with two embodiments of the invention isdescribed with reference to the flow diagram of FIG. 1.

The sequence of steps illustrated in FIG. 1 preferably begins after thedeposition of the resistor and capacitor elements, usually comprisingtantalum or tantalum nitride, on the insulating substrate, which is mostusually alumina. The formation of the interconnection scheme began witha deposition of a layer of titanium over substantially the entire areaof the substrate. The precise method employed was electron gumevaporation, however, other well-known techniques such as sputtering maybe employed. The thickness of the titanium layer is preferably withinthe range 1,500-3,000 A in order to serve as an adequate glue layer andto avoid bondability problems which usually occur when the thickness isless than approximately 1,500 A. A thickness of approximately 2,500 Aseems to be optimum. Next, a thin (approximately 500 A) layer ofpalladium was deposited, preferably by the same technique, on thetitanium layer. This layer serves to improve adhesion between thetitanium and the to be described copper layer. As such, the palladiumlayer is optional since the adhesion of Ti and Cu can be adequate ifproper deposition procedures are followed. An appropriate thickness forthis layer appears to be 200-1,000 A. It should be noted that theincreased adherence of copper and titanium due to the insertion of apalladium layer is quite surprising because a noble metal usually doesnot provide a strong bond with a non-noble metal. The reason for thiseffect is not understood, and is presently being investigated.

Next, a thin layer of copper was deposited, in this embodiment also byelectron gun evaporation, on the Pd layer. The copper layer servesprimarily to provide a high conductivity layer for subsequent platingprocesses. Typically, the layer is approximately 5,000 A in thickness,but a range of approximately 3,000-7,000 A would be appropriate.

Before proceeding with the photoresist step, it is desirable to coat thecopper surface with a thin chrome layer to improve the adherence of thephotoresist. This procedure is well-known in the art. Thephotolithographic process, also well-known in the art, involvesessentially applying a photoresist layer to the entire copper surface,exposing desired areas through a mask, and developing the resist toremove those areas exposed to light in a pattern which will define theappropriate interconnection paths. The particular photoresist used wasmanufactured by Shipley and sold under the trade name AZ-340B, but manyother well-known resists will suffice.

As shown in FIG. 1, a layer of copper was then electroplated onto theselected portions of the evaporated copper layer not covered by thephotoresist. This was accomplished by making the substrate the anode inan electrolytic cell wherein the bath comprised approximately 68gms/liter of CuSO₄ and 180 gms/liter of H₂ SO₄. It was found that thisparticular bath was optimum for complete coverage of the copper layer,but the proportions can be adjusted to fit particular needs. It shouldalso be realized that other baths may be employed for Cu plating, but atthe present time the copper sulphate bath appears optimum. The platingwas carried out at a current density of approximately 20 mA/cm², butagain this parameter may be adjusted depending upon the quantity of thebath and the geometry of the circuit to be fabricated. The thickness ofthe copper layer is an important consideration. In this particularembodiment, the total thickness of evaporated and plated Cu wasapproximately 35,000 A. The appropriate range of thickness inassociation with those of the subsequent Ni and Au layers is discussedlater. An important point to realize is the fact that exposure of thecopper surface to air for prolonged periods causes the surface tooxidize resulting in poor adherence. Therefore, the copper platingoperation should follow the copper evaporation, and the subsequent Niplating should follow the copper plating, while the copper is still wet.

In the next step, nickel was plated onto the exposed areas of the platedcopper surface. The particular bath used for this plating operation issold by Allied-Kelite Co. under the trade name Barrett type SN andbasically comprises nickel sulfamate and boric acid. The thickness ofthe nickel lyer was approximately 10,000 A, although a useful rangeappears to be approximately 8,000-20,000 A for providing an adequatediffusion barrier between the plated copper and the subsequentlydeposited gold layer, while maintaining a proper sheet resistivity asdescribed later. A layer thickness below approximately 8,000 A willresult in a porous film which will not block diffusion of Cu and Au atthe high temperatures required in subsequent thin film processing. Thus,the thickness of this layer is an important parameter. The currentdensity employed was, again, approximately 20 mA/cm² which provides asufficiently dense film. As before, the current density may be adjustedfor particular needs. A minimum current density for the nickel filmfabrication appears to be approximately 10 mA/cm² to produce asufficiently dense film (of the order of 9 gms/cm³).

It was also discovered that an optimum barrier layer is one whichconsists of essentially pure Ni without any additives.

At this point in the processing, as shown in FIG. 1, basically twoalternatives may be followed in forming the top layer of gold. In thefirst alternative, the photoresist layer previously formed on thesubstrate was utilized as the mask for electroplating the gold layer onthe entire exposed area of the previously formed nickel layer. In thesecond alternative, this photoresist layer was stripped off and a secondphotoresist was applied, exposed and developed to expose only thoseareas of the metal which will be utilized as bonding pads for integratedcircuit chips or connection to elements off the substrate. In eitherprocedure, the electroplating processess utilized a gold cyanide bathcomprising 20 gms/liter of potassium gold cyanide, 50 gms/liter ofammonium citrate and 50 gms/liter ammonium sulfate at a current densityof approximately 2 mA/cm². Again, the proportions of the bath componentsand the current density may be adjusted as needed. The thickness of thegold layer was approximately 20,000 A, but it appears that a preferredrange is 15,000-25,000 A to insure a good bonding surface. It will berealized that the thickness of gold required in this conduction systemis considerably less than that required in the previous Ti-Pd-Au systemwhich was approximately 50,000 A. Thus, a substantial cost savings isrealized in accordance with the invention. It will also be realized thatwhile the embodiment involving selective plating in the bonding areassurffers from the disadvantage of requiring an extra photoresist layer,it offers an advantage in requiring less gold and permits further costsavings.

The next processing sequence involves final patterning of theinterconnect scheme by etching the evaporated layers of Cu and Ti whichare not covered by the plated metal layers. Thus, the photoresistutilized during electroplating was first stripped off. In bothalternative embodiments the evaporated copper was removed by an ammoniumpersulfate solution and the titanium layer was subsequently removed byhydrofluoric acid, which solutions are known in the art. Care should betaken in removal of the evaporated copper to avoid nickel etching andundercutting of the plated copper layer. Typically, etch time forremoval of 5,000 A of copper in ammonium persulfate is approximately 60seconds. As a precautionary measure, the circuits were removed from theetchant as soon as all visible signs of copper were removed andimmediately rinsed to stop further etching. It will be realized thatother etchants may be employed for removal of the Ti and Cu layers.

As alluded to previously, the thicknesses of the Cu, Ni, and Au layersare important in providing the proper sheet resistivity for replacementof the Ti-Pd-Au system. In particular, a desired sheet resistance(R_(s)) can be calculated from the equation:

    R.sub.s =  1/[(t/ρ).sub.Au + (t/ρ).sub.Cu + (t/ρ).sub.Ni ](1)

where t is the thickness and ρ the bulk resistivity of the indicatedmetals. An end of life sheet resistance of approximately 0.005ohms/square or less is desired for most applications. Thus, for a Nilayer in the range of 8,000- 20,000 A and Au in the range of15,000-25,000 A, a range of Cu thickness of 25,000-40,000 A appears tobe optimum to satisfy sheet resistance requirements.

After formation of the interconnect pattern as described above, normalcircuit processing such as resistor patterning, thermocompressionbonding, soldering, etc., proceeds in accordance with the prior art.FIG. 2 gives a perspective view of a simple thin film circuit just afterinterconnection formation in accordance with the invention and prior tothe above-mentioned processing. It should be realized that this circuitis presented primarily for illustrative purposes and the presentinterconnect scheme may be utilized for all types of circuits. It willalso be realized that this circuit is not necessarily drawn to scale.

The elements are formed on a ceramic board indicated as 10. Bondingpads, such as 11 formed in the interior define the area of the board 12where the integrated circuit chip (not shown) may be placed. Bondingpads such as 13 formed near the edges permit bonding of the circuit toelements off the board. The circuit shows simply a first resistorelement 22, and a second resistor element 14 in series with a capacitorelement 15, the resistor element usually comprising tantalum nitride andthe capacitor comprising a tantalum-tantalum oxide-conductor multilayerstructure as well-known in the art. The interconnections between bondingpads and circuit elements are made in accordance with the invention. Anenlarged cross-sectional view taken along line 3--3 is illustrated inFIG. 3. The evaporated titanium layer is shown as 16, and the palladiumlayer as 17. The copper layer which was formed by first evaporating athin layer and then electroplating is illustrated as 18 with the platednickel layer formed thereon indicated as 20. It will be seen that the Aulayer 21 is formed only in the areas where bonding is desired inaccordance with the second alternative embodiment previously described.It will be appreciated that during the subsequent heat treatment somealloying of cu, Ni and Au will take place primarily at the boundaries ofthe Ni layer. It is believed that the basic Cu-Ni-Au multilayerstructure will be maintained.

In this regard, one of the significant and surprising features of thismetal interconnect combination is the fact that the combination iscompatible with all and thin film processing sequences required for acomplete circuit in spite of the fact that further processing alterssomewhat the composition of the interconnect structure.

To demonstrate this compatibility, the Ti-Cu-Ni-Au conduction system washeated at various temperatures and times in accordance with normalcircuit processing. For example, film with nickel thicknesses ofapproximately 10,000 A were heated at 250° C for 5 hours, which is thetemperature and time generally used for Ta₂ ZN resistor stabilization.Auger analysis of the resulting structure showed an insignificant amountof diffusion of Cu or Au through the Ni layer and a change in sheetresistance of just 4%. A heat treatment at 150° C for 1,000 hoursresulted in no detectable diffusion through the Ni layer and only a 1.5%change in sheet resistance. An upper limit for heat treatment of thesystem appears to be 350° C for 4 hours, since in this case considerableinterdiffusion took place and sheet resistance change was approximately17%. These tests demonstrated that the Ti-Cu-Ni-Au conduction system wascomparable to Ti-Pd-Au in regard to resistance to damage at the hightemperatures required for processing circuits.

The Ti-Cu-Ni-Au system was also subjected to environments of air plusdry and wet HCl, as well as wet NO₂ and SO₂. In dry HCl, the presentsystem showed the same good corrosion resistance properties as Ti-Pd-Au.In wet environments (HCl, SO₂ or NO₂), which is a more severe test ofresistance to corrosion, the present system showed contact resistancechange approximately equal to that in the dry HCl environment.

Thermocompression bonding to thin film circuits is usually performedafter all high temperature processing is completed. Thus, to testcompatibility of the present system with regard to bonding, it wasnecessary to subject the metals to high temperature for prolongedperiods of time. The system, with Ni layers varying in thickness between2,000-10,000 A was subject to different heat treatments at 150°, 250°,300°, and 350° C, followed by bonding and testing of pull strength.Maximum pull strengths were generally obtained for the 10,000 A Ni layersystem and adequate strengths were observed after treatments of 150° Cup to 1,000 hrs., 250° C up to 10 hours., 300° C up to 4 hrs., and 350°C up to 2 hrs. A minimum time of 30 minutes is suggested. In general,the pull strength for the present conduction system was shown to beessentially the same as the Ti-Pd-Au system similarly treated. The teststhus showed that thermocompression bonding is possible after heatingTi-Cu-Ni-Au at 300° C for 4 hours, which is the treatment currently usedto cure the insulating layer used in plated crossovers and at 350° C forup to 2 hours which is the treatment suggested for resistorstabilization prior to laser trimming, where the Ni layer wasapproximately 10,000 A.

Another surprising feature of the present conduction system was itscompatibility with soldering procedures. Soldering of the Ti-Pd-Ausystem with conventional Sn-Pb solders results in the formation ofbrittle Sn-Au intermetallics. However, with the present system, the goldlayer is sufficiently thin such that no brittle intermetallics areformed. Furthermore, the gold protects the surface of the Ni fromoxidation thus allowing the Ni surface to be wetted readily by thesolder. The dissolution rate of the Ni into the solder is slow enough toinsure adequate time to solder and desolder for making repairs.

Various additional modifications of the present invention will becomeapparent to those skilled in the art. All such variations whichbasically rely on the teachings through which the invention has advancedthe art should properly be considered within the spirit and scope of theinvention.

What is claimed is:
 1. A method of forming electrical interconnectionsof a desired resistivity between thin film elements on an insulatingsubstrate comprising the steps of:forming on the substrate a first metallayer comprising titanium in a desired pattern to a thickness in therange 1,500-3,000 A; forming over said titanium layer a second metallayer comprising copper to a thickness within the range 3,000-7,000 A;electroplating additional copper onto said second layer to produce atotal copper thickness within the range 25,000-40,000 A on selectedareas of said second layer; electroplating a third metal layercomprising nickel over said electroplated copper layer to a thicknesswithin the range 8,000-20,000 A; electroplating a fourth metal layercomprising gold on at least portions of said third layer to a thicknesswithin the range 15,000-25,000 A; and removing those portions of saidfirst and second layers which are not covered by said electroplatedmetals.
 2. The method according to claim 1 wherein the fourth metallayer is formed on substantially the entire surface of said third layer.3. The method according to claim 1 wherein the fourth metal layer isformed only on the portions of said third metal layer which comprise thebonding pads for said circuit.
 4. The method according to claim 1further comprising the step of, prior to formation of said second layer,forming over said first layer a fifth layer comprising palladium.
 5. Themethod according to claim 1 wherein said first and second metal layersare formed by evaporation.
 6. The method according to claim 1 whereinsaid first and second metal layers are formed by sputtering.
 7. Themethod according to claim 1 wherein the additional layer of Cu iselectroplated in a bath comprising CuSO₄ and H₂ SO₄.
 8. The methodaccording to claim 1 wherein said second layer of Cu is removed byimmersing in a solution comprising ammonium persulfate and the firstlayer of Ti is subsequently removed by immersion in a solutioncomprising hydrofluoric acid.
 9. The method according to claim 1 furthercomprising the steps of heating the resulting structure at a temperatureof approximately 250° C for a period of time within the range 30 minutesto 10 hours.
 10. The method according to claim 9 further comprising thestep of heating the resulting structure at a temperature ofapproximately 300° C for a period of time within the range of 30 minutesto 4 hours.
 11. The method according to claim 9 further comprising thestep of heating the resulting structure at a temperature ofapproximately 350° C for a period of time within the range 30 minutes to2 hours.
 12. A method of forming a circuit including thin film elementsand electrical interconnections having a resistivity no greater than0.005 ohms/square on the major surface of an insulating substratecomprising the steps of:evaporating onto substantially the entire majorsurface of said substrate a first metal layer consisting essentially oftitanium to a thickness within the range 1,500-3,000 A; evaporating ontosubstantially the entire surface of said first metal layer a secondmetal layer consisting essentially of palladium to a thickness withinthe range 200-1,000 A; evaporating onto substantially the entire surfaceof said second metal laye a third metal layer consisting essentially ofcopper to a thickness within the range 3,000-7,000 A; electroplatingadditional copper onto selected portions of said evaporated layer ofcopper in the desired interconnection pattern to produce a total Cuthickness within the range 25,000-40,000 A utilizing a plating bathcomprising CuSO₄ and H₂ SO₄ ; while said electroplated copper layer isstill wet, electroplating a fourth metal layer consisting essentially ofnickel on substantially the entire surface of said electroplated copperlayer to a thickness within the range 8,000-20,000 A utilizing a platingbath comprising nickel sulfamate; electroplating onto at least portionsof said fourth metal layer a fifth metal layer consisting essentially ofgold to a thickness within the range 15,000-25,000 A utilizing a platingbath comprising gold cyanide; removing the portions of the evaporatedcopper layer not covered by at least one of said electroplated layers byimmersing in an etching solution comprising ammonium persulfate; andremoving the portions of the evaporated titanium layer not covered by atleast one of said electroplated layers by immersing in an etchingsolution comprising hydrofluoric acid.
 13. The method according to claim12 wherein the fifth layer is formed on substantially the entire surfaceof said fourth layer.
 14. The method according to claim 12 wherein thefifth layer is formed only on the areas of the fourth layer whichcomprise the bonding pads for said circuit.
 15. The method according toclaim 12 further comprising the steps of heating the resulting structureto a temperature of approximately 250° C for approximately 30 minutes to10 hours.
 16. The method according to claim 15 further comprising thestep of heating the structure to a temperature of approximately 300° Cfor approximately 30 minutes to 4 hours.
 17. The method according toclaim 15 further comprising the step of heating the structure to atemperature of approximately 350° C for approximately 30 minutes to 2hours.
 18. A method of forming electrical interconnections of a desiredresistivity between thin film elements on the major surface of aninsulating substrate comprising the steps of:forming on the substrate afirst metal layer comprising titanium in a desired pattern; forming oversaid first metal layer a second metal layer comprising palladium;forming over said second metal layer a third metal layer comprisingcopper; electroplating additional copper onto said third layer onselected areas of said third layer; electroplating a fourth metal layercomprising nickel over said electroplated copper layer; electroplating afifth metal layer comprising gold on at least portions of said fourthlayer; and removing those portions of said first, second and thirdlayers which are not covered by said electroplated metals.
 19. Themethod according to claim 1 wherein the resistivity of the electricalinterconnections is not greater than 0.005 ohms/square.